Existing commercial transport jet aircraft typically include two or more primary turbine engines for propulsion. These aircraft also typically include at least one auxiliary power unit (APU) that provides electrical and/or pneumatic power in addition to or in lieu of the power provided by the primary engines. Accordingly, APUs can be used to provide power to the aircraft when the primary engines are not running, for example, when the aircraft is waiting at an airport gate. The APUs can also provide temporary power to start the primary engines during normal operations, and/or temporary emergency power during an engine-out condition or other emergency condition.
APUs typically include a gas turbine engine that receives air from an APU inlet. FIG. 1 is a partially schematic illustration of an APU 10 housed in an APU compartment 44 in accordance with the prior art. The APU compartment 44 is located within an external fuselage surface 45 of an aircraft. The APU 10 receives air via an inlet 20 that extends from the APU 10 to the external fuselage surface 45. A deployable door 50 opens when the APU 10 is started, and closes when the APU 10 is shut down. The inlet 20 includes a splitter 25 that directs some of the captured air directly to the APU air intake via an APU duct 27, as indicated by arrow A. The remaining air is directed into the APU compartment 44 via an eductor duct 28 (as indicated by arrow E) to ventilate the compartment 44 and to transfer heat from an oil cooler of the APU 10.
While the foregoing arrangement described above with reference to FIG. 1 provides sufficient air to run the APU 10, cool the APU oil, and ventilate the APU compartment 44, the efficiency with which it does so may in some cases not be optimal. Accordingly, it may be desirable to improve the efficiency of the inlet 20 so as to increase the performance of the APU 10, and/or decrease the size and weight of the APU 10.